It is possible, of course, to observe a periodic, high-speed analog signal (hereinafter referred to as a signal to be measured) by a conventional sampling oscilloscope. In general, however, the sampling oscilloscope needs a trigger signal, which bears a certain time relationship, i.e. a synchronous relationship to the signal to be measured, and requires an observer to properly set a delay time, the X-axis level (the time-base) and the Y-axis gain (sensitivity) for displaying the signal to be measured on a CRT for waveform observation and measurement; namely, the conventional sampling oscilloscope has a defect such that operations for the waveform observation and measurement are complex. In particular, for testing whether or not a significant part of the signal to be observed can be observed in a certain time-based range, it is necessary to manually adjust a delay over the entire observable range of delay in the time-based range.
Another prior art example is U.S. Pat. No. 4,000,439, in which a potential of 0.7 volts at the start of a low-speed sawtooth wave is compared with the instantaneous level of an input signal, so that a searched result optimal to observation cannot always be effected for the state of the input signal and the trigger signal.